High-strength stainless steel for mold and preparation method of high-strength stainless steel

Abstract

The invention discloses high-strength stainless steel for a mold. The stainless steel comprises the following chemical elements in percentage by mass: 0-0.04% of C; 0.20% to 0.50% of Si; 0.20% to 0.50% of Mn; p < = 0.005%; s < = 0.002%; cr: 4.50 to 5.50%; 7.80% to 8.20% of Mo; 1.50% to 2.50% of Ni; co: 10.50% to 11.50%; al: less than or equal to 0.015%; ti: less than or equal to 0.005%; h: < = 0.0001%; o: < = 0.0010%; n: less than or equal to 0.0050%; and the balance of Fe and other inevitable impurities. The invention further discloses a preparation method of the high-strength stainless steel for the die. The preparation method comprises the steps of raw material preparation, vacuum induction furnace smelting, preparation before vacuum consumable remelting, vacuum consumable remelting, forging, rolling and heat treatment. The mechanical property of the stainless steel is obviously improved, the service life and the quality level of a manufactured mold can both reach the import standard, and the use cost of the high-strength stainless steel for the mold is effectively reduced.

Description

Technical Field

[0001] This invention relates to the field of stainless steel production technology, and more specifically to a high-strength stainless steel for molds and its preparation method. Background Technology

[0002] High-strength steel refers to steel with a room temperature tensile strength exceeding 1500 MPa and a yield strength greater than 1300 MPa. In addition to a tensile strength of over 1500 MPa, ultra-high-strength steel also requires good plasticity and toughness, minimal notch sensitivity, high fatigue strength, and good processability.

[0003] Ultra-high strength steel, based on carbon-free or low-carbon martensite, undergoes precipitation hardening of intermetallic compounds during aging. Unlike traditional high-strength steel, it is strengthened by the dispersed precipitation of intermetallic compounds instead of carbon. This gives it some unique properties: high strength and toughness, low hardening index, good formability, simple heat treatment process, almost no deformation during aging, and excellent weldability.

[0004] The simplicity of the heat treatment process is a significant advantage of martensitic aging steel. After hot working, the steel should undergo solution treatment before cold working and aging strengthening. The purpose is to dissolve the precipitates remaining after hot working, ensuring sufficient strengthening elements are dissolved in the matrix and obtaining a uniform, high-dislocation-density, fully martensitic structure. After solution treatment, air cooling is performed; the cooling rate has little impact on the microstructure and properties. The high strength of martensitic aging steel is achieved through aging treatment. Air cooling after aging causes a large number of dispersed and ultra-microscopic intermetallic compound particles to precipitate on the martensitic matrix, thereby significantly increasing the material's strength while minimizing toughness loss.

[0005] High-strength steel can be used to manufacture extrusion dies for important aluminum alloy components in aerospace and other industries. The main characteristics of aerospace aluminum alloys include: large size and integral construction, thin walls and lightweight, precise cross-sectional dimensions and geometric tolerances, and uniform and high-quality microstructure and properties. Currently, the service life and quality stability of general die materials available in the domestic market are generally short, restricting the continuity of production and the quality stability of aerospace aluminum alloy products, failing to meet production requirements. Therefore, the aforementioned die materials are heavily reliant on imports from Germany, which are relatively expensive and have long procurement cycles.

[0006] Therefore, there is an urgent need for a high-strength stainless steel for molds and its preparation method. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a high-strength stainless steel for molds and a method for preparing the same, so as to solve the problems in the background art.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows.

[0009] A high-strength stainless steel for molds, wherein the mass percentage of each chemical element in the stainless steel is as follows: C: 0-0.04%; Si: 0.20-0.50%; Mn: 0.20-0.50%; P≤0.005%; S≤0.002%; Cr: 4.50-5.50%; Mo: 7.80-8.20%; Ni: 1.50-2.50%; Co: 10.50-11.50%; Al: ≤0.015%; Ti: ≤0.005%; H: ≤0.0001%; ​​O: ≤0.0010%; N: ≤0.0050%; the balance being Fe and other unavoidable impurities.

[0010] A method for preparing high-strength stainless steel for molds specifically includes the following steps: S1. Prepare raw materials: Select iron-carbon blocks, iron-silicon, metallic chromium, metallic nickel, metallic molybdenum, metallic cobalt, metallic titanium, metallic aluminum, high-purity iron or high-purity steel according to the mass percentage of each chemical element in the above stainless steel. S2, Vacuum induction furnace smelting: S21. Slowly melt the raw materials prepared in step S1 in a vacuum induction furnace at low temperature. The vacuum degree is required to be ≤30Pa. The temperature after complete melting is 1520~1600℃. S22. Refine the melt obtained in step S21 under vacuum degree ≤5Pa and refining temperature of 1530~1570℃ for at least 40min. S23. When the gas contains O≤15ppm, N≤50ppm, and H≤1ppm, Ar≥5000Pa is charged, and then metallic Mn or electrolytic Mn is added to raise the temperature to 1540~1580℃ and stirred evenly to obtain molten steel. S24. Cast the molten steel prepared in step S23 into electrodes. After the electrodes are slowly cooled, machine them and flatten their ends. S3. Preparations before vacuum arc remelting: S31. For each electrode prepared in step S24, assign a consumable steel ingot and cut it flat with the shrinkage cavity facing down. S32. Check whether the electrode blanks entering the consumable furnace are qualified; S33. Install qualified consumable electrodes and dummy electrodes in the crystallizer using in-furnace welding, and ensure L 假电极 +L 电极坯 ≤L 结晶器 +1600mm; S34. Check if the crystallizer is clean; S35. Evacuate the vacuum self-consuming furnace and test for leaks. The ultimate vacuum degree should be ≤0.05Pa, the working vacuum degree should be ≤1.0Pa, and the leakage rate should be ≤1Pa*L / s. S4, Vacuum self-consuming remelting: S41. Turn on helium cooling to accelerate cooling, melting and solidifying simultaneously; S42. The voltage-current control method is adopted, with voltage control at 20-30V, current control at 1-16KA, and arc ignition time ≥40min; S43. The melting rate-droplet control method is adopted, and the melting rate is controlled at 190-400 kg / h, and the droplet rate is 1.5-5.5 drops / s; S44. The melting rate and power are gradually reduced in a stepwise manner. The filling weight of 1-6 ton ingots is 100kg-400kg. After smelting is stopped, the self-consumable steel ingots are cooled in the furnace for 30-60 minutes. S5, Forging: S51. Heat the consumable steel ingot from step S44 to 1100-1160℃ and hold for 4-6 hours. After holding, it is upsetting, drawing and forging twice to form a φ155±10mm intermediate billet forging bar, and then hot-annealed. S52. Heat the intermediate billet forging bar from step S51 to 650-690℃, hold for 15-20h, then cool to 450-500℃ at a rate of ≤50℃ / h and air-cool to room temperature for annealing. S53. After polishing the surface of the annealed intermediate billet forging bar, transfer it to the rolling process; S6, Rolling: S61. Heat the intermediate billet forging bar after surface polishing in step S53 to 1060-1100℃, hold for 1.0-3.0h, roll it into the required φ40-φ75mm specification billet by a 450 rolling mill, and then send it for hot annealing. S62. Heat the rolled bar from step S61 to 650-690℃, hold it for 15-20h, and then cool it to 450-500℃ at a rate of ≤50℃ / h before removing it from the furnace and air cooling it to room temperature. S7. Heat treatment: S71, temperature 1000±20℃, holding time 2.5 mm / min+30min, oil cooling solution treatment; S72, temperature 530±20℃, heat preservation time 2.0 mm / min+120min, air cooling aging.

[0011] To further optimize the technical solution, the raw materials in step S1 are: recycled materials of this steel grade, as well as high-purity pure iron and pure metal materials, wherein the recycled materials of this steel grade account for 10-60% of the raw materials.

[0012] To further optimize the technical solution, the returned material of this steel grade needs to be cleaned or shot blasted to remove rust, oxidation and oil stains from its surface, so as to ensure the cleanliness of the raw materials; the high-purity iron is required to have S≤0.002% and P≤0.005%.

[0013] To further optimize the technical solution, the slow cooling method in step S24 includes sand cooling, hood cooling, pit cooling, and slow cooling with the furnace.

[0014] To further optimize the technical solution, the qualified standard in step S32 is that the surface is free of oxide black skin, peeling skin, black spots and paint; if it is not qualified, it needs to be cleaned by grinding with an angle grinder.

[0015] To further optimize the technical solution, if the crystallizer needs to be cleaned in step S34, an angle grinder with a cloth grinding wheel is used to polish the furnace chamber of the crystallizer to remove the metal adhesion on the furnace wall.

[0016] Due to the adoption of the above technical solutions, the technical progress achieved by this invention is as follows.

[0017] This invention provides a high-strength stainless steel for molds. By selecting superior raw materials and minimizing harmful elements such as P, S, Al, Ti, and gaseous impurities such as H, O, and N, the content of impurities in the steel is reduced, thus improving its purity. Adjusting the chemical composition enhances the interaction between alloying elements, improving the steel's hardness and slightly increasing its ductility and toughness. The manufacturing method employs a dual-vacuum smelting process combining vacuum induction and vacuum self-consumption, which further improves the steel's purity. Reducing residual chemical elements and optimizing the hot working process improves crack sensitivity. The stainless steel of this invention exhibits significantly improved mechanical properties, enabling molds manufactured using this method to meet import standards in terms of service life and quality, effectively reducing the cost of using high-strength stainless steel for molds. Attached Figure Description

[0018] Figure 1 This is a metallographic diagram of the heat-treated structure of Example 1 of the present invention; Figure 2 This is a metallographic diagram of the heat-treated structure of Example 2 of the present invention; Figure 3 This is a metallographic diagram of the heat-treated structure in Example 3 of this invention; Figure 4 This is a metallographic diagram of the heat-treated structure of Comparative Example 1 in this invention; Figure 5 This is a metallographic diagram of the heat-treated structure of Comparative Example 2 in this invention; Detailed Implementation

[0019] A mold made of high-strength stainless steel, combined with Figures 1 to 5As shown, the mass percentage of each chemical element in stainless steel is as follows: C: 0~0.04%; Si: 0.20~0.50%; Mn: 0.20~0.50%; P≤0.005%; S≤0.002%; Cr: 4.50~5.50%; Mo: 7.80~8.20%; Ni: 1.50~2.50%; Co: 10.50~11.50%; Al: ≤0.015%; Ti: ≤0.005%; H: ≤0.0001%; ​​O: ≤0.0010%; N: ≤0.0050%; the balance is Fe and other unavoidable impurities.

[0020] The design principle of the steel grade described in this invention is as follows: As an age-hardening ultra-high strength stainless steel, this invention improves the purity of the material by selecting superior raw materials and reducing the content of impurity elements, thereby improving crack sensitivity; by adjusting the chemical composition and rationally adding the content of alloying elements, the interaction between alloying elements is improved, thereby increasing the hardness of the steel and slightly improving its ductility and toughness.

[0021] Considering the limitations of current metallurgical technology, within the scope of production capacity, this invention aims to minimize the content of harmful elements such as C, P, S, Al, Ti, and gaseous impurity elements such as H, O, and N, in order to improve the purity of the steel and reduce its crack sensitivity.

[0022] Cr element: In this steel, its role is to improve the corrosion resistance of stainless steel, stabilize ferrite, lower the Ms point (the starting temperature of martensite transformation, which refers to the critical driving force temperature required for the phase transformation when the difference in free energy between the austenite and martensite phases reaches the threshold), and improve hardenability.

[0023] Mo (Mo) is an alloying element beneficial to both strength and toughness in this steel. In the early stages of aging, it precipitates molybdenum-rich phases, strengthening the steel while maintaining its toughness. The molybdenum precipitates along the original austenite grain boundaries, thus preventing intergranular fracture and improving fracture toughness. However, the addition of excessive molybdenum (more than 10%), like excessive nickel, will generate retained austenite, relatively reducing the amount of molybdenum-rich precipitates and weakening the strengthening effect.

[0024] Ni: In this steel, as an element that expands the austenite phase region, it can strongly inhibit the transformation of austenite to pearlite and bainite during the cooling process after austenitization, thereby improving the hardenability of the steel. Nickel also lowers the temperature at which supercooled austenite transforms into martensite, increases the content of retained austenite, and is beneficial to toughness. In addition, it can increase the stacking fault energy of the steel matrix, making it easier for screw dislocations to cross-slip, thereby improving toughness.

[0025] Co (Co) element: In this steel, it reduces the solid solubility of Mo in martensite, increases the martensitic transformation point, and thus promotes the precipitation of Mo-containing intermetallic compounds (such as Ni3Mo and Fe2Mo). Simultaneously, Co inhibits the recovery of dislocation substructures in martensite, providing more nucleation sites for subsequent precipitate formation, resulting in finer and more uniformly distributed precipitate particles and reduced interparticle spacing. Co also increases the Mo solution temperature, partially compensating for the adverse effect of Ni, ensuring that the steel, after solution treatment, is entirely martensitic—a function that other elements cannot replace. The large addition of Co raises the martensitic transformation point, resulting in a higher martensite content at room temperature, which increases the steel's strength without cryogenic treatment. Furthermore, the large addition of Co simplifies the heat treatment process, eliminating the need for cryogenic treatment.

[0026] Al and Ti elements: Al and Ti readily combine with nitrogen (N) in steel to form AlN and TiN, increasing the amount of E-type nonmetallic inclusions and leading to a decrease in the strength and toughness of the steel. To achieve the theoretical matrix strength of the steel and improve its purity, the content of Al and Ti must be strictly controlled to be as low as possible.

[0027] P and S elements: In this steel, P reduces cold bending performance and worsens weldability. While P increases strength, hardness, wear resistance, and corrosion resistance, these positive effects are usually offset by the cold brittleness it causes. P dissolves in ferrite, and due to its large atomic diameter, it causes ferrite lattice distortion, significantly reducing the steel's plasticity and toughness, especially at low temperatures. Furthermore, P tends to segregate during steel crystallization, which is difficult to eliminate through heat treatment, increasing the risk of cold brittleness. S hazards: S easily causes brittle fracture during hot working of steel, a phenomenon known as hot brittleness. The presence of S also reduces the steel's impact toughness, fatigue strength, weldability, and corrosion resistance. Even trace amounts of S are harmful to steel, therefore, its content must be strictly controlled. S forms brittle compounds in the weld, leading to weak weld joints and even cracking during welding. S also reduces the cold working properties of steel and affects its plastic deformation capacity, making the steel more prone to cracking and fracture during bending, stamping, and other forming processes. The presence of P and S in steel has a negative impact on its physical and processing properties, therefore, their content in steel needs to be strictly controlled.

[0028] Therefore, harmful elements such as P, S, Al, Ti and gases such as H, O, N are all considered as impurity elements. The raw materials undergo a dual vacuum smelting process of vacuum induction and vacuum self-consumption remelting to reduce the content of impurity elements to the lowest possible level and achieve high purity.

[0029] This invention also discloses a method for preparing high-strength stainless steel for molds, specifically including the following steps: S1. Prepare raw materials: Select iron-carbon blocks, iron-silicon, metallic chromium, metallic nickel, metallic molybdenum, metallic cobalt, metallic titanium, metallic aluminum, high-purity iron, or high-purity steel according to the mass percentage of each chemical element in the above stainless steel.

[0030] Raw materials can also include recycled materials of this steel grade, as well as high-purity iron and pure metal materials. Recycled materials of this steel grade account for 10-60% of the raw materials, and these recycled materials need to undergo surface treatment methods such as cleaning or shot blasting to remove surface rust, oxidation, and oil stains, ensuring the cleanliness of the raw materials. High-purity iron requires S≤0.002% and P≤0.005%.

[0031] S2, Vacuum induction furnace smelting: S21. Slowly melt the raw materials prepared in step S1 in a vacuum induction furnace at a low temperature. The vacuum degree is required to be ≤30Pa, and the temperature after full melting is 1520~1600℃.

[0032] S22. Refine the melt obtained in step S21 under vacuum degree ≤5Pa and refining temperature of 1530~1570℃ for at least 40min.

[0033] S23. When the gas contains O≤15ppm, N≤50ppm, and H≤1ppm, Ar≥5000Pa is charged, and then metallic Mn or electrolytic Mn is added to raise the temperature to 1540~1580℃ and stirred evenly to obtain molten steel.

[0034] S24. Cast the molten steel prepared in step S23 into electrodes. After the electrodes are slowly cooled, machine them and flatten their ends. The slow cooling methods include sand cooling, hood cooling, pit cooling, and slow cooling with the furnace.

[0035] S3. Preparations before vacuum arc remelting: S31. For each electrode prepared in step S24, assign a consumable steel ingot, and cut it flat with the shrinkage cavity facing down.

[0036] S32. Check whether the electrode blanks entering the consumable furnace are qualified; the qualified standard is that there is no oxide black skin, no peeling skin, no black spots and paint on the surface; if they are not qualified, they need to be cleaned by grinding with an angle grinder.

[0037] S33. Install qualified consumable electrodes and dummy electrodes in the crystallizer using in-furnace welding, and ensure L 假电极 +L 电极坯 ≤L 结晶器 +1600mm.

[0038] S34. Check if the crystallizer is clean; if the crystallizer needs to be cleaned, use an angle grinder with a cloth grinding wheel to polish the furnace chamber and remove the metal adhering to the furnace wall.

[0039] S35. Evacuate the vacuum self-consuming furnace and test for leaks. The ultimate vacuum degree should be ≤0.05Pa, the working vacuum degree should be ≤1.0Pa, and the leakage rate should be ≤1Pa*L / s.

[0040] S4, Vacuum self-consuming remelting: S41. Turn on helium cooling to accelerate cooling, melting and solidifying simultaneously.

[0041] S42. The voltage-current control method is adopted, with voltage control at 20-30V, current control at 1-16KA, and arc ignition time ≥40min.

[0042] S43. The melting rate-droplet control method is adopted, and the melting rate is controlled at 190-400 kg / h, and the droplet rate is 1.5-5.5 drops / s.

[0043] S44. The melting rate and power are gradually reduced in a stepwise manner. The filling weight of 1-6 ton ingots is 100kg-400kg. After smelting is stopped, the self-consuming steel ingots are cooled in the furnace for 30-60 minutes.

[0044] S5, Forging: S51. Heat the consumable steel ingot from step S44 to 1100-1160℃ and hold for 4-6 hours. After holding, forge it into a φ155±10mm intermediate billet forging bar by upsetting and drawing twice, and then send it for hot annealing.

[0045] S52. Heat the intermediate billet forging bar from step S51 to 650-690℃, hold for 15-20h, then cool to 450-500℃ at a rate of ≤50℃ / h, and then air-cool to room temperature for annealing.

[0046] S53. After polishing the surface of the annealed intermediate billet forging bar, transfer it to the rolling process.

[0047] S6, Rolling: S61. Heat the intermediate billet forging bar after surface polishing in step S53 to 1060-1100℃, hold for 1.0-3.0h, roll it into the required φ40-φ75mm specification billet using a 450 rolling mill, and then send it for hot annealing.

[0048] S62. Heat the rolled bar from step S61 to 650-690℃, hold it at that temperature for 15-20 hours, and then cool it to 450-500℃ at a rate of ≤50℃ / h before removing it from the furnace and air-cooling it to room temperature.

[0049] S7. Heat treatment: S71, temperature 1000±20℃, holding time 2.5 mm / min+30min, oil cooling solution treatment.

[0050] S72, temperature 530±20℃, heat preservation time 2.0 mm / min+120min, air cooling aging.

[0051] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Example

[0052] A high-strength stainless steel for molds, wherein the mass percentage of each chemical element in the stainless steel is: C=0.02%; Si=0.32%; Mn=0.38%; P=0.003%; S=0.0015%; Cr=4.75%; Mo=8.00%; Ni=1.65%; Co=10.80%; Al=0.012%; Ti=0.003%; H=0.00008%; O=0.0008%; N=0.0035%; the balance being Fe and other unavoidable impurities.

[0053] A method for preparing high-strength stainless steel for molds specifically includes the following steps: S1. Prepare raw materials: Select iron-carbon blocks, iron-silicon, metallic chromium, metallic nickel, metallic molybdenum, metallic cobalt, metallic titanium, metallic aluminum, high-purity iron, or high-purity steel according to the mass percentage of each chemical element in the above stainless steel.

[0054] S2, Vacuum induction furnace smelting: S21. Slowly melt the raw materials prepared in step S1 at low temperature in a vacuum induction furnace, with a vacuum degree of 10 Pa and a temperature of 1520℃ after complete melting.

[0055] S22. The melt obtained in step S21 is refined for 50 minutes under a vacuum of 1 Pa and a refining temperature of 1540℃.

[0056] S23. When the gas contains O≤15ppm, N≤50ppm, and H≤1ppm, Ar is charged at 8000Pa, and then metallic Mn or electrolytic Mn is added to raise the temperature to 1560℃ and stirred evenly to obtain molten steel.

[0057] S24. Cast the molten steel prepared in step S23 into electrodes. After the electrodes are slowly cooled, machine them and flatten their ends. The slow cooling methods include sand cooling, hood cooling, pit cooling, and slow cooling with the furnace.

[0058] S3. Preparations before vacuum arc remelting: S31. For each electrode prepared in step S24, assign a consumable steel ingot, and cut it flat with the shrinkage cavity facing down.

[0059] S32. Check whether the electrode blanks entering the consumable furnace are qualified; the qualified standard is that there is no oxide black skin, no peeling skin, no black spots and paint on the surface; if they are not qualified, they need to be cleaned by grinding with an angle grinder.

[0060] S33. Install qualified consumable electrodes and dummy electrodes in the crystallizer using in-furnace welding, and ensure L 假电极 +L 电极坯 ≤L 结晶器 +1600mm.

[0061] S34. Check if the crystallizer is clean; if the crystallizer needs to be cleaned, use an angle grinder with a cloth grinding wheel to polish the furnace chamber and remove the metal adhering to the furnace wall.

[0062] S35. Evacuate the vacuum self-consuming furnace and test for leaks. The ultimate vacuum degree should be ≤0.05Pa, the working vacuum degree should be ≤1.0Pa, and the leakage rate should be ≤1Pa*L / s.

[0063] S4, Vacuum self-consuming remelting: S41. Turn on helium cooling to accelerate cooling, melting and solidifying simultaneously.

[0064] S42. The voltage-current control method is adopted, with the voltage control being 25V, the current control being 12KA, and the arc ignition time being 40min.

[0065] S43. The melting rate-droplet control method is adopted, and the melting rate is controlled at 250 kg / h and the droplet rate is 2.5 drops / s.

[0066] S44. The melting rate and power are gradually reduced in a stepwise manner, and the filling weight of 1-ton to 6-ton ingots is 150kg; after smelting is stopped, the self-consumable steel ingots are cooled in the furnace for 40 minutes.

[0067] S5, Forging: S51. Heat the consumable steel ingot from step S44 to 1150℃ and hold for 5 hours. After holding, it is forged into a φ155±10mm intermediate billet forging bar by upsetting and drawing twice, and then sent for hot annealing.

[0068] S52. Heat the intermediate billet forging bar from step S51 to 670°C, hold for 18 hours, then cool to 450°C at a rate of ≤50°C / h, and then air-cool to room temperature for annealing.

[0069] S53. After polishing the surface of the annealed intermediate billet forging bar, transfer it to the rolling process.

[0070] S6, Rolling: S61. The intermediate billet forging bar after surface polishing in step S53 is heated to 1080℃, held for 2.0h, rolled into the required φ50mm specification billet by a 450 rolling mill, and then sent for hot annealing.

[0071] S62. Heat the rolled bar from step S61 to 670°C, hold it for 15 hours, and then cool it to 500°C at a rate of ≤50°C / h before removing it from the furnace and air-cooling it to room temperature.

[0072] S7. Heat treatment: S71, temperature 990℃, holding time 2.5 mm / min + 30min, oil cooling solution treatment.

[0073] S72, temperature 520℃, holding time 2.0 mm / min + 120min, air-cooled aging. Example

[0074] A high-strength stainless steel for molds, wherein the mass percentage of each chemical element in the stainless steel is: C=0.03%; Si=0.2%; Mn=0.5%; P=0.002%; S=0.002%; Cr=5.5%; Mo=8.20%; Ni=1.5%; Co=10.50%; Al=0.015%; Ti=0.004%; H=0.00008%; O=0.0006%; N=0.0035%; the balance being Fe and other unavoidable impurities.

[0075] A method for preparing high-strength stainless steel for molds specifically includes the following steps: S1. Prepare raw materials: Select iron-carbon blocks, iron-silicon, metallic chromium, metallic nickel, metallic molybdenum, metallic cobalt, metallic titanium, metallic aluminum, high-purity iron, or high-purity steel according to the mass percentage of each chemical element in the above stainless steel.

[0076] S2, Vacuum induction furnace smelting: S21. Slowly melt the raw materials prepared in step S1 in a vacuum induction furnace at a low temperature. The vacuum degree is 30 Pa, and the temperature after complete melting is 1600℃.

[0077] S22. The melt obtained in step S21 is refined for 60 minutes under a vacuum of 3 Pa and a refining temperature of 1530℃.

[0078] S23. When the gas contains O≤15ppm, N≤50ppm, and H≤1ppm, Ar is charged at 7000Pa, and then metallic Mn or electrolytic Mn is added to raise the temperature to 1540℃ and stirred evenly to obtain molten steel.

[0079] S24. Cast the molten steel prepared in step S23 into electrodes. After the electrodes are slowly cooled, machine them and flatten their ends. The slow cooling methods include sand cooling, hood cooling, pit cooling, and slow cooling with the furnace.

[0080] S3. Preparations before vacuum arc remelting: S31. For each electrode prepared in step S24, assign a consumable steel ingot, and cut it flat with the shrinkage cavity facing down.

[0081] S32. Check whether the electrode blanks entering the consumable furnace are qualified; the qualified standard is that there is no oxide black skin, no peeling skin, no black spots and paint on the surface; if they are not qualified, they need to be cleaned by grinding with an angle grinder.

[0082] S33. Install qualified consumable electrodes and dummy electrodes in the crystallizer using in-furnace welding, and ensure L 假电极 +L 电极坯 ≤L 结晶器 +1600mm.

[0083] S34. Check if the crystallizer is clean; if the crystallizer needs to be cleaned, use an angle grinder with a cloth grinding wheel to polish the furnace chamber and remove the metal adhering to the furnace wall.

[0084] S35. Evacuate the vacuum self-consuming furnace and test for leaks. The ultimate vacuum degree should be ≤0.05Pa, the working vacuum degree should be ≤1.0Pa, and the leakage rate should be ≤1Pa*L / s.

[0085] S4, Vacuum self-consuming remelting: S41. Turn on helium cooling to accelerate cooling, melting and solidifying simultaneously.

[0086] S42. The voltage-current control method is adopted, with the voltage control being 20V, the current control being 1KA, and the arc initiation time being 50min.

[0087] S43. The melting rate-droplet control method is adopted, with the melting rate controlled at 190 kg / h and the droplet count at 5.5 drops / s.

[0088] S44. The melting rate and power are gradually reduced in a stepwise manner, and the filling weight of 1-ton to 6-ton ingots is 400kg; after smelting is stopped, the self-consumable steel ingots are cooled in the furnace for 30 minutes.

[0089] S5, Forging: S51. Heat the consumable steel ingot from step S44 to 1160℃ and hold for 4 hours. After holding, it is forged into a φ155±10mm intermediate billet forging bar by upsetting and drawing twice, and then sent for hot annealing.

[0090] S52. Heat the intermediate billet forging bar from step S51 to 690°C, hold for 15 hours, then cool to 480°C at a rate of ≤50°C / h, and then air-cool to room temperature for annealing.

[0091] S53. After polishing the surface of the annealed intermediate billet forging bar, transfer it to the rolling process.

[0092] S6, Rolling: S61. The intermediate billet forging bar after surface polishing in step S53 is heated to 1060℃, held for 1.0h, rolled into the required φ40mm specification billet by a 450 rolling mill, and then sent for hot annealing.

[0093] S62. Heat the rolled bar from step S61 to 650°C, hold it for 20 hours, and then cool it to 450°C at a rate of ≤50°C / h before removing it from the furnace and air-cooling it to room temperature.

[0094] S7. Heat treatment: S71, temperature 1020℃, holding time 2.5 mm / min + 30min, oil cooling solution treatment.

[0095] S72, temperature 510℃, holding time 2.0 mm / min + 120min, air-cooled aging. Example

[0096] A high-strength stainless steel for molds is produced using recycled material from Example 1, accounting for approximately 20%, with the remainder made from high-purity iron or high-purity steel and pure metal materials that meet the requirements. The mass percentage of each chemical element in the stainless steel is as follows: C=0.04%; Si=0.5%; Mn=0.2%; P=0.001%; ​​S=0.001%; ​​Cr=4.5%; Mo=7.80%; Ni=2.5%; Co=11.50%; Al=0.013%; Ti=0.005%; H=0.00009%; O=0.0008%; N=0.002%; the balance being Fe and other unavoidable impurities.

[0097] A method for preparing high-strength stainless steel for molds specifically includes the following steps: S1. Prepare raw materials: Select iron-carbon blocks, iron-silicon, metallic chromium, metallic nickel, metallic molybdenum, metallic cobalt, metallic titanium, metallic aluminum, high-purity iron, or high-purity steel according to the mass percentage of each chemical element in the above stainless steel.

[0098] S2, Vacuum induction furnace smelting: S21. Slowly melt the raw materials prepared in step S1 at low temperature in a vacuum induction furnace, with a vacuum degree of 20 Pa and a temperature of 1550℃ after complete melting.

[0099] S22. The melt obtained in step S21 is refined for 45 minutes under a vacuum of 5 Pa and a refining temperature of 1570℃.

[0100] S23. When the gas contains O≤15ppm, N≤50ppm, and H≤1ppm, Ar is charged at 6000Pa, and then metallic Mn or electrolytic Mn is added to raise the temperature to 1580℃ and stirred evenly to obtain molten steel.

[0101] S24. Cast the molten steel prepared in step S23 into electrodes. After the electrodes are slowly cooled, machine them and flatten their ends. The slow cooling methods include sand cooling, hood cooling, pit cooling, and slow cooling with the furnace.

[0102] S3. Preparations before vacuum arc remelting: S31. For each electrode prepared in step S24, assign a consumable steel ingot, and cut it flat with the shrinkage cavity facing down.

[0103] S32. Check whether the electrode blanks entering the consumable furnace are qualified; the qualified standard is that there is no oxide black skin, no peeling skin, no black spots and paint on the surface; if they are not qualified, they need to be cleaned by grinding with an angle grinder.

[0104] S33. Install qualified consumable electrodes and dummy electrodes in the crystallizer using in-furnace welding, and ensure L 假电极 +L 电极坯 ≤L 结晶器 +1600mm.

[0105] S34. Check if the crystallizer is clean; if the crystallizer needs to be cleaned, use an angle grinder with a cloth grinding wheel to polish the furnace chamber and remove the metal adhering to the furnace wall.

[0106] S35. Evacuate the vacuum self-consuming furnace and test for leaks. The ultimate vacuum degree should be ≤0.05Pa, the working vacuum degree should be ≤1.0Pa, and the leakage rate should be ≤1Pa*L / s.

[0107] S4, Vacuum self-consuming remelting: S41. Turn on helium cooling to accelerate cooling, melting and solidifying simultaneously.

[0108] S42. The voltage-current control method is adopted, with the voltage control at 30V, the current control at 16KA, and the arc ignition time at 60min.

[0109] S43. The melting rate-droplet control method is adopted, with the melting rate controlled at 400 kg / h and the droplet count at 1.5 drops / s.

[0110] S44. The melting rate and power are gradually reduced in a stepwise manner, and the filling weight of 1-ton to 6-ton ingots is 100kg; after smelting is stopped, the self-consumable steel ingots are cooled in the furnace for 60 minutes.

[0111] S5, Forging: S51. Heat the consumable steel ingot from step S44 to 1100℃ and hold for 6 hours. After holding, it is forged into a φ155±10mm intermediate billet forging bar by upsetting and drawing twice, and then sent for hot annealing.

[0112] S52. Heat the intermediate billet forging bar from step S51 to 650°C, hold for 20 hours, then cool to 500°C at a rate of ≤50°C / h, and then air-cool to room temperature for annealing.

[0113] S53. After polishing the surface of the annealed intermediate billet forging bar, transfer it to the rolling process.

[0114] S6, Rolling: S61. The intermediate billet forging bar after surface polishing in step S53 is heated to 1100℃, held for 3.0h, rolled into the required φ75mm specification billet by a 450 rolling mill, and then sent for hot annealing.

[0115] S62. Heat the rolled bar from step S61 to 690°C, hold it for 18 hours, and then cool it to 470°C at a rate of ≤50°C / h before removing it from the furnace and air-cooling it to room temperature.

[0116] S7. Heat treatment: S71, temperature 980℃, holding time 2.5 mm / min + 30min, oil cooling solution treatment.

[0117] S72, temperature 550℃, holding time 2.0 mm / min + 120min, air-cooled aging.

[0118] The difference between this application and Example 1 is that the mass percentage of each chemical element in the stainless steel is different. In step S51 of the preparation process, the consumable steel ingot does not undergo two upsetting processes and is directly drawn and forged into an intermediate billet forging bar.

[0119] The difference between this application and Example 2 is that the mass percentage of each chemical element in the stainless steel is different, and the solution temperature in step S7 of the preparation process is 1100℃.

[0120] The stainless steels prepared in Examples 1-3 and Comparative Examples 1-2 were tested respectively. The metallographic structures in the heat-treated state are shown in the appendix. Figure 1-5 Then, the mechanical properties of the above-mentioned stainless steels were tested, and the results are shown in the table below. Hardness / HRC U-impact / J Yield / MPa Tensile strength / MPa elongation / % Shrinkage / % Example 1 50.5 12.2 1645 1986 12.4 50 Example 2 51.0 10.3 1747 1957 11.6 45 Example 3 51.7 10.0 1795 1981 10.8 42 Comparative Example 1 50.2 8.1 1326 1456 9.5 37 Comparative Example 2 50.1 7.8 1414 1501 9 33 As can be seen from the images, the microstructure of the stainless steel prepared by the method of this application is better after heat treatment. In Comparative Example 1, there are carbides distributed in strips, which reduces the mechanical properties of stainless steel. In Comparative Example 2, the microstructure is obviously coarsened, which is not as good as the example.

[0121] In summary, the raw materials used in this application undergo a dual vacuum smelting process involving vacuum induction and vacuum arc remelting to minimize harmful elements such as P, S, Al, and Ti, as well as gaseous impurities such as H, O, and N, achieving high purity. The stainless steel prepared by this method exhibits a yield strength exceeding 1640 MPa, a tensile strength exceeding 1950 MPa, an elongation exceeding 10%, and a reduction of area exceeding 40%. The mechanical properties of the stainless steel prepared by this method are significantly improved. This low-cost, high-strength, and highly corrosion-resistant stainless steel can be used in the manufacture of critical structures in shipbuilding, marine engineering, and aerospace engineering.

Claims

1. A high-strength stainless steel for molds, characterized in that, The mass percentage of each chemical element in stainless steel is as follows: C: 0–0.04%; Si: 0.20–0.50%; Mn: 0.20–0.50%; P≤0.005%; S≤0.002%; Cr: 4.50–5.50%; Mo: 7.80–8.20%; Ni: 1.50–2.50%; Co: 10.50–11.50%; Al: ≤0.015%; Ti: ≤0.005%; H: ≤0.0001%; ​​O: ≤0.0010%. N: ≤0.0050%; balance is Fe and other unavoidable impurities.

2. A method for preparing high-strength stainless steel for molds as described in claim 1, characterized in that, Specifically, the following steps are included: S1. Prepare raw materials: Select iron-carbon blocks, iron-silicon, metallic chromium, metallic nickel, metallic molybdenum, metallic cobalt, metallic titanium, metallic aluminum, high-purity iron or high-purity steel according to the mass percentage of each chemical element in the above stainless steel. S2, Vacuum induction furnace smelting: S21. Slowly melt the raw materials prepared in step S1 in a vacuum induction furnace at low temperature. The vacuum degree is required to be ≤30Pa. The temperature after complete melting is 1520~1600℃. S22. Refine the melt obtained in step S21 under vacuum degree ≤5Pa and refining temperature of 1530~1570℃ for at least 40min. S23. When the gas contains O≤15ppm, N≤50ppm, and H≤1ppm, Ar≥5000Pa is charged, and then metallic Mn or electrolytic Mn is added to raise the temperature to 1540~1580℃ and stirred evenly to obtain molten steel. S24. Cast the molten steel prepared in step S23 into electrodes. After the electrodes are slowly cooled, machine them and flatten their ends. S3. Preparations before vacuum arc remelting: S31. For each electrode prepared in step S24, assign a consumable steel ingot and cut it flat with the shrinkage cavity facing down. S32. Check whether the electrode blanks entering the consumable furnace are qualified; S33. Install qualified consumable electrodes and dummy electrodes in the crystallizer using in-furnace welding, and ensure L 假电极 +L 电极坯 ≤L 结晶器 +1600mm; S34. Check if the crystallizer is clean; S35. Evacuate the vacuum self-consuming furnace and test for leaks. The ultimate vacuum degree should be ≤0.05Pa, the working vacuum degree should be ≤1.0Pa, and the leakage rate should be ≤1Pa*L / s. S4, Vacuum self-consumable remelting: S41. Turn on helium cooling to accelerate cooling, melting and solidifying simultaneously; S42. The voltage-current control method is adopted, with voltage control at 20-30V, current control at 1-16KA, and arc ignition time ≥40min; S43. The melting rate-droplet control method is adopted, and the melting rate is controlled at 190-400 kg / h, and the droplet rate is 1.5-5.5 drops / s; S44. The melting rate and power are gradually reduced in a stepwise manner. The filling weight of 1-6 ton ingots is 100kg-400kg. After smelting is stopped, the self-consumable steel ingots are cooled in the furnace for 30-60 minutes. S5, Forging: S51. Heat the consumable steel ingot from step S44 to 1100-1160℃ and hold for 4-6 hours. After holding, it is upsetting, drawing and forging twice to form a φ155±10mm intermediate billet forging bar, and then hot-annealed. S52. Heat the intermediate billet forging bar from step S51 to 650-690℃, hold for 15-20h, then cool to 450-500℃ at a rate of ≤50℃ / h and air-cool to room temperature for annealing. S53. After polishing the surface of the annealed intermediate billet forging bar, transfer it to the rolling process; S6. Rolling: S61. Heat the intermediate billet forging bar after surface polishing in step S53 to 1060-1100℃, hold for 1.0-3.0h, roll it into the required φ40-φ75mm specification billet by a 450 rolling mill, and then send it for hot annealing. S62. Heat the rolled bar from step S61 to 650-690℃, hold it for 15-20h, and then cool it to 450-500℃ at a rate of ≤50℃ / h before removing it from the furnace and air cooling it to room temperature. S7. Heat treatment: S71, temperature 1000±20℃, holding time 2.5 mm / min+30min, oil cooling solution treatment; S72, temperature 530±20℃, heat preservation time 2.0 mm / min+120min, air cooling aging.

3. The method for preparing high-strength stainless steel for molds according to claim 2, characterized in that, The raw materials in step S1 are: recycled materials of this steel grade, as well as high-purity pure iron and pure metal materials, wherein the recycled materials of this steel grade account for 10-60% of the raw materials.

4. The method for preparing high-strength stainless steel for molds according to claim 3, characterized in that: The returned material of this steel grade needs to be cleaned or shot blasted to remove rust, oxidation and oil stains from its surface, so as to ensure the cleanliness of the raw materials; the high-purity iron requires S≤0.002% and P≤0.005%.

5. The method for preparing high-strength stainless steel for molds according to claim 2, characterized in that: The slow cooling methods in step S24 include sand cooling, hood cooling, pit cooling, and slow cooling with the furnace.

6. The method for preparing high-strength stainless steel for molds according to claim 2, characterized in that: The acceptable standard in step S32 is that the surface is free of oxide black skin, peeling skin, black spots, and paint; if it is not acceptable, it needs to be cleaned by grinding with an angle grinder.

7. The method for preparing high-strength stainless steel for molds according to claim 2, characterized in that: If the crystallizer needs to be cleaned in step S34, the furnace chamber of the crystallizer is polished with an angle grinder equipped with a cloth grinding wheel to remove the metal adhering to the furnace wall.

Images